The development of the retinotopic projection to the optic tectum in the chick embryo seems to occur on the basis of highly specific attraction between axons of retinal ganglion cells and complementary tectal neurons. Previous experiments show that the retinal ganglion cells acquire positional specificities early in development, but acquisition of complementary specificities by tectal neurons has not been confirmed. It seems possible that specified ganglion cell axons could orient along an electrical or chemical gradient on a tectal surface which was non-specified. In the proposed study, partial tectal ablations are to be performed at different times in the chick to show if tectal cell affinities are determined, and if so, at what time. If tectal cell affinities are rigidly determined at a particular time, ganglion cells complementary to ablated areas could be identified by their subsequent retrograde degeneration in the retina. Should the remaining tectum exhibit plasticity by forming a new set of specificities or a gradient which was not locus specific, ganglion cells from all retinotopic positions could form connections. The primary objectives of this study are to use the light and electron microscope 1) to determine if ganglion cells can form connections in abnormal tectal, or even non-tectal areas, 2) to evaluate several methods in embryos that might offer good resolution of such abnormal pathways and terminations, and 3) to study retrograde degeneration in ganglion cells that have not made connections.